"In the last few days, you may have seen a blog indicating that “other graphics APIs such as OpenGL and AMD’s Mantle are not available” on at least one of the next-generation consoles. Starting with our public introduction in Hawaii, we clearly stated that Mantle is designed for the PC platform, where it creates a development environment that is similar to what consoles already offer: low-level APIs, close-to-metal hardware access, and simplified development procedures versus that of a PC. PC gamers and developers deserve the benefits of this model as well, which is why devs like DICE approached us and requested a technology like Mantle. And the benefit of that technology is clear: improved performance for gamers through more efficient rendering.

So much of the work game developers are doing to prepare for the next generation of console gaming is already well-suited for the modern graphics architectures in AMD Radeon graphics cards. Though the door is open for non-PC platforms to support Mantle in the future, today Mantle is a continuum that allows developers to take advantage of that work on the PC." - AMD PR

So, it appears that Microsoft not supporting AMD Mantle won't cause the world to end. It is highly likely that even is console game developers don't use the full Mantle API that they will be writing code that they will be optimizing code for AMD's Graphics Core Next (GCN) in the first place and that should carry over when console games are ported over to the PC. This should play in AMD's favor and Mantle lays the ground work for low-level access to hardware for game developers. AMD also said again that DICE approached them for this ability in the first place. The real question now is how many game developers will be fully using the Mantle API.